merge-index: fix longstanding bug in merging symlinks

merge-index: fix longstanding bug in merging symlinks

Ancient commit e2b6a9d0 added code to pass "file modes" from
merge-index to merge-one-file, and then later commit 54dd99a1
wanted to make sure we do not end up creating a nonsense symlink
that points at a path whose name contains conflict markers.

However, nobody noticed that the code in merge-index added by
e2b6a9d0 were stripping the S_IFMT bits and the code in 54dd99a1
was meaningless.  This fixes it.

Signed-off-by: Junio C Hamano <junkio@cox.net>